From: Rong Tao <[email protected]>
In commit b1fc58333575 ("stop_machine: Avoid potential race behaviour")
fix both multi_cpu_stop() and set_state() access multi_stop_data::state,
Pass curstate as a parameter to ack_state(), to avoid the non-atomic read.
And replace smp_wmb()+WRITE_ONCE() with smp_store_release().
Signed-off-by: Rong Tao <[email protected]>
---
v1: stop_machine: Avoid potential race behaviour of multi_stop_data::state
https://lore.kernel.org/lkml/[email protected]/
---
kernel/stop_machine.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index cedb17ba158a..35a122ce2cbd 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -183,15 +183,15 @@ static void set_state(struct multi_stop_data *msdata,
{
/* Reset ack counter. */
atomic_set(&msdata->thread_ack, msdata->num_threads);
- smp_wmb();
- WRITE_ONCE(msdata->state, newstate);
+ smp_store_release(&msdata->state, newstate);
}
/* Last one to ack a state moves to the next state. */
-static void ack_state(struct multi_stop_data *msdata)
+static void ack_state(struct multi_stop_data *msdata,
+ enum multi_stop_state curstate)
{
if (atomic_dec_and_test(&msdata->thread_ack))
- set_state(msdata, msdata->state + 1);
+ set_state(msdata, curstate + 1);
}
notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
@@ -242,7 +242,7 @@ static int multi_cpu_stop(void *data)
default:
break;
}
- ack_state(msdata);
+ ack_state(msdata, curstate);
} else if (curstate > MULTI_STOP_PREPARE) {
/*
* At this stage all other CPUs we depend on must spin
--
2.42.0
On Thu, Oct 19, 2023 at 08:11:23AM +0800, Rong Tao wrote:
> From: Rong Tao <[email protected]>
>
> In commit b1fc58333575 ("stop_machine: Avoid potential race behaviour")
> fix both multi_cpu_stop() and set_state() access multi_stop_data::state,
> Pass curstate as a parameter to ack_state(), to avoid the non-atomic read.
Can we please describe this better? This is *not* a fix, it is a cleanup.
As I covered in:
https://lore.kernel.org/lkml/ZS5g6I-UtUnihToH@FVFF77S0Q05N/
... there are no concurrent writers, and so the value of multi_stop_data::state
cannot change, and a non-atomic read is fine.
The actual change looks good to me as it makes it easier to see that there's no
race.
> And replace smp_wmb()+WRITE_ONCE() with smp_store_release().
This is also fine, but feels like a logically separate change.
Mark.
>
> Signed-off-by: Rong Tao <[email protected]>
> ---
> v1: stop_machine: Avoid potential race behaviour of multi_stop_data::state
> https://lore.kernel.org/lkml/[email protected]/
> ---
> kernel/stop_machine.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index cedb17ba158a..35a122ce2cbd 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -183,15 +183,15 @@ static void set_state(struct multi_stop_data *msdata,
> {
> /* Reset ack counter. */
> atomic_set(&msdata->thread_ack, msdata->num_threads);
> - smp_wmb();
> - WRITE_ONCE(msdata->state, newstate);
> + smp_store_release(&msdata->state, newstate);
> }
>
> /* Last one to ack a state moves to the next state. */
> -static void ack_state(struct multi_stop_data *msdata)
> +static void ack_state(struct multi_stop_data *msdata,
> + enum multi_stop_state curstate)
> {
> if (atomic_dec_and_test(&msdata->thread_ack))
> - set_state(msdata, msdata->state + 1);
> + set_state(msdata, curstate + 1);
> }
>
> notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
> @@ -242,7 +242,7 @@ static int multi_cpu_stop(void *data)
> default:
> break;
> }
> - ack_state(msdata);
> + ack_state(msdata, curstate);
> } else if (curstate > MULTI_STOP_PREPARE) {
> /*
> * At this stage all other CPUs we depend on must spin
> --
> 2.42.0
>
On Fri, Oct 20, 2023 at 11:23:37AM +0100, Mark Rutland wrote:
> > And replace smp_wmb()+WRITE_ONCE() with smp_store_release().
>
> This is also fine, but feels like a logically separate change.
Agreed, and please take this opportunity to write a comment with the
barrier.